I. Field of the Invention
This invention relates to a radiographic intensifying screen.
II. Description of the Prior Art
Radiographic photographing is usually carried out by overlapping a photosensitive film and a pair of radiographic intensifying screens. The radiographic intensifying screen comprises a support, a phosphor layer formed on the support and a comparatively thin, transparent protective film formed on the phosphor layer. The phosphor layer consists of fine phosphor particles and a binding agent for holding these particles together.
When taking a radiographic picture, the photosensitive film is sandwiched between a pair of radiographic intensifying screens with the transparent protective film of the radiographic intensifying screen in contact with the photosensitive film. The laminated system is put in a cassette, and is irradiated with radioactive rays from outside of the cassette. When radioactive rays are incident on the phosphor of the radiographic intensifying screen, fluorescence is produced. This fluorescence reaches the photosensitive film to sensitize the film.
It has been desired that as much light as possible be emitted from the phosphor layer and should irradiate the photosensitive film with a high degree of sharpness. To this end, it has been proposed to vary the grain size of the phosphor particles constituting the phosphor layer in the direction of thickness of the phosphor layer, as disclosed in, for instance, U.S. patent specification No. 4,039,840. According to this patent, phosphor particles are arranged such that the grain size of the particles increases from the support side of the radiographic intensifying screen toward the protective film side thereof.
By the provision of such a grain size distribution, the modulation transfer function of the intensifying screen is promoted since the light path of the reflection and diffusion, in the phosphor layer, of the fluorescent light emitted from the phosphor is shortened and therefore more fluorescent light can be taken up from the surface of the phosphor layer. In the structure of the phosphor layer having such a grain size distribution, however, the selection of the grain size gives large influence to the modulation transfer function of the radiographic intensifying screen. For example, when the average grain size of the large phosphor particles in the surface (the side from which emitted fluorescent light is taken up) of the phosphor layer is extremely large, or when the grain size of the small phosphor particles in the support side of the phosphor layer is not so different from that of the large phosphor particles in the surface of the phosphor layer, the modulation transfer function of the radiographic intensifying screen is not improved.